private void PdfViewerElement_DocumentUnloaded(object sender, EventArgs e)
{
    RadPdfViewerContainer container = this.PdfViewer.PdfViewerElement.Parent as RadPdfViewerContainer;
    if (container != null)
    {
        foreach (var node in container.BookmarksTree.TreeViewElement.GetNodes())
        {
            node.Tag = null;
        }

        container.BookmarksTree.Nodes.Clear();
    }

    typeof(RadPdfViewerElement).GetField("previousPage", BindingFlags.NonPublic | BindingFlags.Instance).
                                 SetValue(this.PdfViewer.PdfViewerElement, null);

}

// If running on .NET 4.5.1 and later
// https://docs.microsoft.com/en-us/dotnet/api/system.runtime.gcsettings.largeobjectheapcompactionmode?view=net-5.0#System_Runtime_GCSettings_LargeObjectHeapCompactionMode
GCSettings.LargeObjectHeapCompactionMode = GCLargeObjectHeapCompactionMode.CompactOnce;
//

GC.Collect();
GC.WaitForPendingFinalizers();
GC.WaitForFullGCApproach();
GC.WaitForFullGCComplete();
GC.Collect();